a For each molecule, the calculated
Vmax or Vmin value refers to the atom
marked with asterisk in the table.
b ΔVmin refers to the changes
of the most negative MEP (Vmin) on the free N atom in
the binary complex relative to that of the corresponding monomer
(CN-Ph-CN* or Br-Ph-CN*).
As shown in Figure 1, the two CN substituents in CN-Ph-CN make it to be
a typical Lewis base due to its withdrawing nature. When R is
substituted by Br atom, the MEPs maps of Br-Ph-CN possess a σ-hole on
the extensions of the C-Br bond, which indicates that Br-Ph-CN can act
as both Lewis acid and Lewis base. The Vmax and
Vmin in these isolated monomers are gathered in Table 4.
For the halogen-containing compounds, there are small positive
electrostatic potential caps (σ-holes) on the outermost portion along
the molecular axis, and these σ-holes represent the potential
interaction sites with the Lewis base CN-Ph-CN. The calculated
Vmax values for the halogen compounds ranges from 12.99
to 54.37 kcal/mol. For dihalogen compounds, the σ-hole values become
more positive in the order
F2<BrCl*<Cl2<Br2<ClBr*<FCl*<FBr*,
and the atom which interact with CN-Ph-CN is denoted by an asterisk. As
for BH3 and BF3, two positive
electrostatic potential regions exist along the vertical direction of
the molecular plane (π-hole), corresponding to the location of the emptyp orbital of the B atom, and thus a favorable triel bond with a
Lewis base is expected for BH3 and BF3.
The value of Vmax is larger in BF3 than
that in BH3, which is in agreement with the earlier
studies [44]. For
the four nitrogen bases, blue regions with negative MEPs are found on
the surface of the N atom, and the values of Vmin,Nincrease in the order
HCN(sp )<NH2CH3(sp 3)
<(≈)NHCH2(sp 2)<NH3(sp 3),
which is in accordance with the result obtained by Li et al .[57]
3.2.2 Geometries and Interactions of the Ternary
complexes
Here, we will discuss the issue that when introducing the halogen or
triel bond into the original binary complex, how the pnicogen bond will
be affected by the interplay of these interactions. The optimized
structures of some representative ternary complexes are plotted in
Figure 8. The geometrical parameters and the interaction energies of the
ternary and binary complexes are summarized in Table 5 and Table S1,
respectively. For the trimolecular complexes
X…CN-Ph-CN…PO2F, when X is the halides
compounds (F2, Cl2, Br2,
FCl, FBr, BrCl, ClBr, FCN, ClCN, BrCN), the free N atom in CN-Ph-CN
interacts with the halides along the extension of C-N bond, forming
halogen bonds, and when X is the boron-containing compound
(BH3, BF3), the B…N(C) triel
bonds will be formed. As for the ternary complexes of
Y…Br-Ph-CN…PO2F, the Br atom interacts
with the N-bases denoted by Y, including NH3,
NH2CH3, NHCH2, and HCN,
with the formation of the N…Br halogen bond. The geometry of the
P…N pnicogen bond is similar to that in the binary complexes, and
the only difference is the P…N binding distance and the F-P-O-O
dihedral angle, which indicate the strength of the pnicogen bond and the
geometrical deformation upon formation of the ternary complexes,
respectively.